Because of their excellent mechanical strength, heat resistance, transparency and gas barrier properties, polyesters such as polyethylene terephthalate are favorably used as not only materials of containers of various beverages such as juice, soft drinks and carbonated beverages but also materials of films, sheets and fibers.
The polyesters can be generally produced using, as starting materials, dicarboxylic acids such as aromatic dicarboxylic acids and diols such as aliphatic diols. In more detail, a dicarboxylic acid and a diol are first subjected to esterification reaction to form a low condensate (low molecular weight polyester), and the low condensate is then subjected to deglycolation reaction (liquid phase polycondensation) to increase the molecular weight. In some cases, solid phase polycondensation is performed to further increase the molecular weight.
In the process for producing polyesters mentioned above, a conventional antimony compound, a conventional germanium compound or the like is used as a polycondensation catalyst.
However, the polyester produced by the use of the antimony compound as a polycondensation catalyst is inferior to the polyester produced by the use of a germanium compound as a polycondensation catalyst in the transparency and the heat resistance. In the use of the antimony compound as a polycondensation catalyst, further, the acetaldehyde content in the resulting polyester is desired to be decreased. On the other hand, the germanium compound is considerably expensive, so that the production cost of polyester becomes high. To decrease the production cost, a process including recovering the germanium compound scattered during the polycondensation and reusing it has been studied.
By the way, it is known chat titanium is an element having a function of promoting polycondensation reaction of a low condensate. Titanium compounds such as titanium alkoxide, titanium tetrachloride, titanyl oxalate and orthotitanic acid are publicly known as polycondensation catalysts, and various studies have been made to utilize such titanium compounds as the polycondensation catalysts.
However, when the conventional titanium compounds are used as the polycondensation catalysts, their activity is inferior to that of the antimony compound or the germanium compounds. In addition, the resulting polyester has a problem of being markedly colored yellow, and hence they have not been put into practical use yet. In the industrial production of polyesters using these titanium compounds as the polycondensation catalysts, further, there is a problem of corrosion caused by elution of chlorine content in case of catalysts containing a large amount of chlorine, such as titanium tetrachloride and partial hydrolyzate of titanium tetrachloride. Therefore, catalysts having low chlorine content are sometimes desired.
Under such circumstances as described above, catalysts for polyester production capable of producing polyesters with high polycondensation activity or catalysts for polyester production capable of producing such polyesters as satisfy any one of requirements of low acetaldehyde content, high transparency and excellent tint with high catalytic activity are desired.
There are also desired a process for producing polyesters by which polyesters having desired intrinsic viscosity (IV) can be obtained for a short period of time, a process for producing polyesters by which polyesters having low acetaldehyde content can be obtained with high polymerization activity, and a process for producing polyesters by which polyesters having excellent tint can be obtained with high polymerization activity.
As described above, the polyester, particularly polyethylene terephthalate, is favorably used as a material of containers of beverages such as juice, soft drinks and carbonated beverages.
To produce a blow molded article from the polyester, the polyester is fed to a molding machine such as an injection molding machine to form a preform for a blow molded article, then the preform is inserted in a mold of a given shape, and the preform is subjected to stretch blow molding and a heat treatment (heat setting)
As for the molded product obtained from the conventional polyester such as conventional polyethylene terephthalate, however, the content of acetaldehyde is increased during the molding and the acetaldehyde remains in the resulting molded product, so that flavor or scent of the contents filled in the molded product is sometimes considerably deteriorated
As a process for producing polyethylene terephthalate having small increase of the acetaldehyde content during the molding, a process, which includes treating a particulate polyethylene terephthalate with water vapor of 110° C. or higher prior to solid phase polycondensation of the polyethylene terephthalate, is disclosed in Japanese Patent Laid-Open Publication No 25815/1984, or a process for producing polyethylene terephthalate of high polymerization degree, which includes a step of moisture controlling polyethylene terephthalate having an intrinsic viscosity of not less than 0.4 dl/g and a density of not more than 1.35 g/cm3 to vary the moisture content to not less than 0.2% by weight, a step of precrystallizing the polyethylene terephthalate at a temperature of 140° C. or higher, and a step of solid phase polymerization at a temperature of 180 to 240° C. in an inert gas atmosphere or reduced pressure, is disclosed in Japanese Patent Laid-Open Publication No. 219328/1984.
However, the increase of the acetaldehyde content in the polyethylene terephthalate obtained by these processes cannot be lowered down below a certain level.
In Japanese Patent Laid-Open Publication No. 97990/ 1993, a method for treating polyethylene terephthalate comprising bringing pellets of polyethylene terephthalate having been subjected to solid phase polymerization into contact with a phosphoric acid aqueous solution having a concentration of not less than 1 ppm is disclosed.
In this method, however, the phosphoric acid functions as an aced catalyst to perform hydrolysis, and as a result, decrease of the intrinsic viscosity is accelerated during the melt molding.
The conventional polyester, e.g., polyethylene terephthalate, contains oligomers such as a cyclic trimer, and the oligomers such as a cyclic trimer adhere to an inner surface of a mold for blow molding or a gas exhaust vent or a gas exhaust pipe of a mold to cause stain of the mold, or adhere to a vent zone of an injection molding machine. The stain of the mold causes surface roughening or whitening of the resulting blow molded article. The whitened blow molded article must be discarded. In the production of a blow molded article using the conventional polyester, the stain of the mold must be frequently removed, and this results in conspicuous lowering of productivity of the blow molded article.
In addition, the polyester obtained by the use of the antimony compound or the germanium compound as a polycondensation catalyst sometimes has low melt flowability and is insufficient in the moldability.
Under such circumstances as described above, there is desired a polyester having a low acetaldehyde content, hardly increased in the acetaldehyde content during the molding and hardily causing stain of a mold or a polyester having high melt flowability and excellent moldability.
Further, there is also desired a polyester molded product having excellent transparency and tint or a molded product such as a blow molded article preform or a blow molded article, e.g., a polyester blow molded article having a low content of a cyclic primer.
One of the present applicants has found that the main cause of the stain of a mold in the molding process resides in that large amounts of oligomers such as a cyclic trimer are produced in the molding of the polyester to increase the total amount of the oligomers such as a cyclic trimer contained in the polyester, and has also found that the increase of the oligomers such as a cyclic trimer can be remarkably inhibited by bringing polyester obtained through the solid phase polycondensation into contact with water of the like, so they have proposed this in Japanese Patent Laid-Open Publication No. 283393/1996